3D scanning is well known per se. Non-contact, hand-held scanners are available on the market and can be used to obtain a 3D representation of an immobile object by moving the hand-held scanner around the object, that is relative to a reference located at a fixed position on the ground (ground reference), while maintaining the field of view of the scanner oriented toward the object. Non-contact scanning can be done either actively or passively. Active scanning involves emitting a light signal or the like which is reflected on the surface to be scanned, whereas passive scanning does not control emission and can rather involve a process similar to detecting depth using human binocular vision. Hand-held scanners are typically surface scanners. Surface scanning is different from solid scanning in the sense that in surface scanning only information about the surface of the scanning subject is obtained, to form a shell model of the object, whereas in solid scanning, information about the subject (typically density), is obtained at various depths across it, such as in X-ray computed tomography for instance (CT).
With a hand-held scanner, for most situations, a single scan taken from a single position and orientation of the scanner in space will not produce a complete model of the subject. Multiple scans, even hundreds, from many different directions are usually required to obtain information about all sides of the subject. These scans each produce a depth image which have to be brought in a common reference system, a process that will be referred to herein as registration, to create a complete model inclusive of hidden faces in individual ones of the depth images. The depth images typically include at least a depth value for each pixel corresponding to the field of view of the scanner, so the depth images can be 3D images if the scanner has a 2D array of pixels.
Although known hand-held scanner based scanning systems and methods were satisfactory to a certain degree, there remained room for improvement. In particular, there remained applications where the step of registration met difficulties. One example lies in instances where the scanning system “lost” the common reference system and then became unable to register later images with earlier images. This is a particular challenge when dealing with live subjects which can move or change its shape during scanning.